The present invention relates to methods and compositions for increasing the viscosity of a treatment fluid, and, more specifically, to methods and compositions for treating a subterranean formation using a viscosified treatment fluid that contains a polymeric metal crosslinking agent.
Many industrial applications, including those in the upstream energy industry, utilize viscosified fluids or “viscosified treatment fluids.” As used herein, the terms “treatment” or “treating” refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. The terms “treatment” and “treating,” as used herein, do not imply any particular action by the fluid or any particular component thereof. Treatment fluids can include, for example, drilling fluids, fracturing fluids, gravel packing fluids, acidizing fluids, conformance treatment fluids, damage control fluids, remediation fluids, scale removal and inhibition fluids, chemical floods, and the like. Generally, viscosified treatment fluids that are used in subterranean operations are aqueous-based fluids that comprise gelling agents. These gelling agents can be biopolymers or synthetic polymers. Common gelling agents that can be used in viscosified treatment fluids can include, for example, galactomannan gums, cellulosic polymers, and polysaccharides.
Most viscosified treatment fluids crosslink the gelling agent using a crosslinking agent to increase the fluid's viscosity. Common crosslinking agents can comprise a metal ion, a transition metal, or a metalloid, which are collectively referred to herein as “metal(s).” Illustrative metals suitable for crosslinking can include, for example, aluminum, antimony, zirconium, magnesium and titanium, chromium. Generally, the metal of a crosslinking agent can interact with at least two gelling agent molecules to form a crosslink between them, thereby forming a crosslinked gelling agent.
Although conventional metal crosslinking agents can frequently be used in viscosified treatment fluids, the use of such crosslinking agents can be problematic because they may not form a viscoelastic gel below a critical concentration of gelling agent (e.g., the critical overlap concentration C*). In addition, such viscosified treatment fluids may not be thermally stable at high temperatures (e.g., temperatures exceeding about 300° F.), such that a loss of viscosity occurs over time. To offset these types of viscosity losses, the concentration of the gelling agent and/or the crosslinking agent can be increased, albeit at an increased cost of goods. Also, these viscosified treatment fluids may lack shear tolerance. Further, higher concentrations of the gelling agent and/or the crosslinking agent can make the viscosified treatment fluid more difficult to remove from a subterranean formation.
Accordingly, an ongoing need exists for methods of crosslinking gelling agents that lowers the required metal concentration and improves the shear sensitivity of the treatment fluid.